Piezoelectric sensor and input device comprising same

ABSTRACT

A piezoelectric sensor arranged so as to includes: a transparent piezoelectric element having a piezoelectric property; and a pair of transparent conductor film layers opposed to each other with the piezoelectric element therebetween, the transparent piezoelectric element and the transparent conductor film layers are formed between a pair of transparent substrates, opposed to each other, which serve as pressure transmission means. Consequently, the transparent piezoelectric sensor has an excellent durability. A piezoelectric sensor comprises a piezoelectric element with a piezoelectric property which is made of a piezoelectric material having no Curie point and has a dipole orientation degree of not less than 75%. Consequently, the piezoelectric sensor having an excellent durability and a simple structure is provided at low cost.

TECHNICAL FIELD

The present invention relates to a piezoelectric sensor and an inputdevice including the same. More particularly, the present inventionrelates to a piezoelectric sensor for detecting physical quantities suchas acoustic emissions, pressure, oscillation, and acceleration in ahigh-temperature environment such as an inside of an internal combustionengine (or an inside of an internal combustion cylinder) and an insideof a plant such as an atomic power plant, or to a transparentpiezoelectric sensor having a piezoelectric element formed betweentransparent conductor films opposite to each other (transparentelectrodes), and to a transparent input device having a plurality of thepiezoelectric sensors.

BACKGROUND ART

Conventionally, piezoelectric sensors have been employed in variousfields.

As an example, there is a transparent input device using a piezoelectricsensor. Examples of the transparent input device in use are aresistance-film transparent input device (Japanese Laid-Open PublicationNo. 242759/1993 (Tokukaihei 5-242759; published on Sep. 21, 1993)), acapacitance transparent input device (Japanese Laid-Open Publication No.324203/1993 (Tokukaihei 5-324203; published on Dec. 7, 1993)), ananalog-capacitor transparent input device, anultrasonic-surface-elastic-wave transparent input device (e.g., JapaneseLaid-Open Publication No. 182842/2002 (Tokukai 2002-182842; published onJun. 28, 2002)), and an infrared-scanning transparent input device(e.g., Japanese Laid-Open Publication No. 45155/1999 (Tokukai 11-45155;published on Feb. 16, 1999)).

The resistance-film transparent input device is constituted of a pair ofsubstrates opposite to each other, i.e., a top sheet (upper substrate)on a surface of an input panel and a lower substrate opposed thereto.Further, an inside of the upper substrate is coated with a transparentconductor film, and an inside of the lower substrate is also coated witha transparent conductor film. The transparent conductor films areopposed to each other at a predetermined distance. That is, thetransparent conductor films are out of contact with each other.

Meanwhile, the capacitance transparent input device and theanalog-capacitor transparent input device, as with the resistance-filmtransparent input device, detect a contact position based on such aprinciple that a capacitance changes when a finger touches an inputpanel coated with a transparent conductor film layer.

Further, the ultrasonic-surface-elastic-wave transparent input deviceand the infrared-scanning transparent input device detect a contactposition by scanning a surface of a transparent panel with a surfaceelastic wave and infrared light respectively.

In case of the resistance-film transparent input device, when the topsheet is subjected to outside pressure, the transparent conductor filmsdisposed at a distance from each other are caused to come into contactwith each other. The contact position is calculated from a voltagegradient of the transparent conductor films for detection. Therefore, inthe resistance-film transparent input device, the top sheet needs to bedeformed by being subjected to outside pressure. As a result, theresistance-film transparent input device raises such problems that: thetransparent conductor films are flawed by coming into contact with eachother, and durability is impaired because the transparent conductorfilms need to be deformed.

Meanwhile, the capacitance transparent input device and theanalog-capacitor transparent input device detect a contact positionaccording to a capacitance change. Therefore, the capacitancetransparent input device and the analog-capacitor transparent inputdevice raise such problem that they may malfunction when anelectromagnetic noise is generated.

Moreover, the ultrasonic-surface-elastic-wave transparent input deviceand the infrared-scanning transparent input device raise such problemthat they tend to have a complex structure and have difficulty indealing with simultaneous multipoint contact.

Accordingly, a piezoelectric sensor and a transparent input device havebeen demanded which have an excellent durability and an anti-noiseproperty.

Further, as an example of other applications of a piezoelectric sensor,there is a piezoelectric sensor which is installed in a high-temperaturestructure such as a pipe and a valve in a plant (e.g., an atomic powerplant) or an internal-combustion engine in order to detect anabnormality in the structure. For example, an acoustic emission sensorand a piezoelectric oscillation sensor have been used. The acousticemission sensor detects an acoustic emission, i.e., an elastic wavewhich is generated when the structure is cracked and broken. Thepiezoelectric oscillation sensor detects an abnormal oscillation andinformation on acceleration. These sensors come in various types, suchas a compression type, a cantilever type, a diaphragm type, and a sheartype.

Among them, a compression-type thin-film piezoelectric sensor including:a laminated body having a pedestal with a pedestal-side electrode; apiezoelectric body; a load-body-side electrode; and a load body, whereinthese materials are laminated in this order, and the compression-typethin-film piezoelectric sensor is used with a lower surface of thepedestal strongly, i.e., firmly mounted on a target object. When anoscillation occurs in the target object, the oscillation is transmittedto the pedestal side of the sensor. Whereas the pedestal side of thesensor oscillates together with the target object, the load body sideoscillates with delay due to inertial force, and the piezoelectric bodyis subjected to a compressive stress or tensile stress proportional tooscillatory acceleration. Further, a potential or a voltage proportionalto the stress is generated on both sides of the piezoelectric body andis extracted (taken out) by the two electrodes (the pedestal-sideelectrode and the load-body-side electrode). A measurement of theelectrical output so extracted makes it possible to detect a size of theoscillation or the acceleration of the target object.

Conventionally, a piezoelectric body made of a piezoelectric materialsuch as lead zirconate titanate and vinylidene polyfluoride, asdescribed in Japanese Laid-Open Publication No. 148011/1994 (Tokukaihei6-148011; published on May 27, 1994) and Japanese Laid-Open PublicationNo. 206399/1998 (Tokukaihei 10-206399; published on Aug. 7, 1998), hasbeen used for such a piezoelectric sensor. However, a piezoelectric bodymade of such a piezoelectric material has a low Curie point (the term“Curie point” means a temperature at which a polarization of such apiezoelectric body disappears.) and therefore has a maximum operatingtemperature of about 300° C. at the highest. Accordingly, in order tokeep a temperature of a piezoelectric body at an applicable temperature,Japanese Laid-Open Publication No. 203665/1993 (Tokukaihei 5-203665;published on Aug. 10, 1993) discloses a piezoelectric body cooled with apeltiert element. However, because the peltiert element only has afunction of simply generating a local temperature gradient, a coolingmechanism cannot be mounted on an outside remote from the piezoelectricbody, so that the peltiert element cannot be applied to a part where awhole of the piezoelectric body becomes hot.

Therefore, as described above, because the conventional thin-filmpiezoelectric sensor cannot withstand high temperatures, an oscillationof a target object which reaches a high temperature is brought throughan oscillation transmission bar to a remote low-temperature environmentfor measurement. However, an oscillation such as an acoustic emission isattenuated due to a property of an oscillation transmission substance inthe process or is mixed with an external redundant oscillation in theprocess of transmission, so that the oscillation of the target objectcannot be measured sufficiently accurately. That is, for the purpose ofan accurate measurement, it is desirable that an oscillation be measuredin a place as proximate as possible to the place where the oscillationhas occurred.

This is achieved by a thin-film piezoelectric sensor, disclosed inJapanese Laid-Open Publication No. 34230/1993 (Tokukaihei 5-34230;published on Feb. 9, 1993), which withstands high temperatures and whosepiezoelectric layer is made of a piezoelectric material such as lithiumniobate, which has a high Curie point. Lithium niobate has a Curie pointof about 1140° C. and can be used in a high-temperature environmentwithout cooling means. However, lithium niobate is hard to make thinnerand needs to be a monocrystalline body to obtain a piezoelectricproperty, thereby raising such problem that it is difficult to produceand process the sensor at low cost.

A high-temperature thin film oscillation sensor described in JapaneseLaid-Open Publication No. 122948/1998 (Tokukaihei 10-122948; publishedon May 15, 1998), in order to solve these problems, is arranged so thatzinc oxide or aluminum nitride is used as a piezoelectric ceramic havingno Curie point, and a thin film including the piezoelectric ceramicoriented in a c-axis direction is used as a piezoelectric thin filmelement.

However, a substance whose crystal has a wurtzite structure (e.g., zincoxide and aluminum nitride, described in the foregoing patent document)has difficulty in retaining a piezoelectric property, and it isimpossible to stably improve a piezoelectric property only by a c-axisorientation of an axis of the crystal. That is, a c-axis orientation isa factor necessary to improve a piezoelectric property but is notsufficient by itself to stably retain a piezoelectric property.Experiment data shows that even when a substance having an excellentpiezoelectric property can be produced, the substance is notreproducible. In some cases, a piezoelectric property is not expressedat all.

This is because a piezoelectric sensor produced by the method of theforegoing patent document has a substrate and a piezoelectric layerprovided directly thereon and therefore cannot stably align a directionof a dipole of a crystal of a piezoelectric element. Even when apiezoelectric element which has a high dipole orientation degree isproduced, it is difficult to obtain a piezoelectric element whosepiezoelectric layer has a high dipole orientation degree. Specifically,it is impossible to cause the piezoelectric layer to keep a dipoleorientation degree not less than 75%. This prevents the piezoelectricsensor from retaining a piezoelectric property and causes such problemthat pressure cannot be detected satisfactorily.

Accordingly, a small, inexpensive thin-film piezoelectric sensor fordetecting an acoustic emission and an oscillation or acceleration hasbeen demanded which ensures a piezoelectric property by thinning apiezoelectric material having no Curie point and orienting a polarity ofa crystal in the thin film, requires no cooling means, and has anexcellent durability.

Moreover, such conditions are required in a cylinder internal-pressuresensor for grasping a phenomenon in a combustion chamber of an internalcombustion engine. Conventionally, the cylinder internal-pressuresensor, disposed on an inner surface of a cylinder, transmits internalpressure of the cylinder through a diaphragm and a pressure transmissionbar to a piezoelectric element, and extracts from the piezoelectricelement an electrical signal proportional to a size of the internalpressure of the cylinder, so that the pressure is detected. Theforegoing piezoelectric element is generally a piezoelectric elementmade of a ceramic material such as lead zirconate titanate and leadtitanate.

However, as with an ignition plug, a piezoelectric sensor which directlymeasures internal pressure of a cylinder is exposed to a high combustiontemperature (500° C.), and a piezoelectric element reaches a very hightemperature (about 400° C.).

A ceramic piezoelectric element made of lead zirconate titanate has aCurie point of about 250° C.; a ceramic piezoelectric element made oflead titanate has a Curie point of about 350° C. These temperatures areboth lower than the foregoing combustion temperature and undesirablyallow the piezoelectric elements to reach their respective Curie points.When a piezoelectric material reaches a high temperature exceeding aCurie point, a piezoelectric element exhibits a deterioration in apiezoelectric property due to depolarization and the like and thereforebecomes unusable, so that the piezoelectric element is usually used incombination with separate cooling means for keeping a temperature of thepiezoelectric element at a suitable temperature.

Meanwhile, an arrangement requiring no cooling means may be achieved bya piezoelectric element disclosed in Japanese Laid-Open Publications No.34230/1993, as described already, and Japanese Laid-Open Publication No.180286/2000 (Tokukai 2000-180286; published on Jun. 30, 2000). Thepiezoelectric element is made of a monocrystalline piezoelectricmaterial (e.g., lithium niobate) which has a relatively high Curiepoint. Lithium niobate has a Curie point of about 1140° C. Therefore,even when the piezoelectric element reaches a high temperature of about400° C. in case of measuring internal pressure of a cylinder, thepiezoelectric element, having a much higher Curie point, does notdeteriorate, thereby requiring no cooling means.

However, lithium niobate, having a low processability, is hard to makethinner and needs to be used in a monocrystalline state. Moreover,because a special method is required to form lithium niobate into anarbitrary shape, lithium niobate is limited in handling and thereforecauses a problem with cost.

Further, lithium niobate has a problem with retention of a monocrystal.When a monocrystal of lithium niobate is brought into direct contactwith a diaphragm and the diaphragm is subjected to uneven pressure, anelectrode, disposed on an opposite side of the diaphragm, which servesto retain the monocrystal, is distorted. In the worst case, a retentionpart may be damaged. In order to prevent this, a bar-like pressuretransmission mechanism for transmitting internal pressure of a cylinderto a piezoelectric element is required. However, this inevitably resultsin a complex structure.

For example, Japanese Laid-Open Publication No. 34230/1993 discloses apressure sensor which has a detection element and a diaphragm. Thedetection element is constituted of a piezoelectric element, a pressuretransmission mechanism, and the like, and is stored in an inside of amain metal body mounted in a sensor-mounting screw hole provided in acylinder block. Also, the diaphragm is press-fitted onto a lower endsurface of the main metal body facing a cylinder. However, a pressuretransmission bar needs to be provided between the diaphragm and thepiezoelectric element.

Further, in Japanese Laid-Open Publication No. 180286/2000, a pressuretransmission bar is not used, but a diaphragm is provided with aprojection. A piezoelectric element has a load-receiving structure whichgenerates a compressive stress so that a pressure detection element isnot deflected (bent) even under a load due to internal pressure of acylinder from the projection of the diaphragm.

Thus, the conventional piezoelectric materials cause a pressuretransmission structure to be complex, large and expensive and thereforecannot satisfy demand.

Accordingly, in view of this, an inexpensive piezoelectric sensor havingan excellent durability and a simple structure has been demanded.

The present invention, completed in consideration of the foregoingproblems, has a first object to provide a piezoelectric sensor, made ofa transparent pressure-sensitive material having a piezoelectricproperty, which has an excellent durability and an anti-noise property,and a transparent input device including the same.

Further, the present invention has a second object to provide a small,inexpensive piezoelectric sensor, ensuring a piezoelectric property,requiring no cooling means, and having an excellent durability, whichdetects an acoustic emission and an oscillation or acceleration, ordetects internal pressure of a cylinder in order to grasp a phenomenonin a combustion chamber of an internal combustion engine.

DISCLOSURE OF INVENTION

As a result of various studies of a method of forming a transparent thinfilm made of a transparent electrode with a piezoelectric materialthereon, the inventors have found that the foregoing objects can beachieved by depositing a piezoelectric ceramic in a monocrystallineshape to form a thin film, and have completed the present inventionbased on that knowledge.

That is, the present invention provides a piezoelectric sensor and atransparent input device including a plurality of the piezoelectricsensors, the piezoelectric sensor including an electrode constituted ofa transparent conductor film, a transparent piezoelectric ceramicmonocrystalline thin film being formed on the electrode, a transparentconductor film being provided on the thin film, an electric circuitbeing provided in the two conductor layers via detection means.

In order to solve the foregoing problems, the piezoelectric sensoraccording to the present invention is arranged so as to include: atransparent piezoelectric element having a piezoelectric property; and apair of transparent conductor film layers opposed to each other with thepiezoelectric element therebetween, the transparent piezoelectricelement and the transparent conductor film layers being formed between apair of transparent substrates, opposed to each other, which serve aspressure transmission means.

In other words, the piezoelectric sensor of the present invention isarranged so that a layer made of a piezoelectric element having apiezoelectric property is formed between transparent conductor films ina pair of substrates having the transparent conductor films.

With the foregoing arrangement, when one of the pair the transparentsubstrates is subjected to outside pressure, the pressure acts on thepiezoelectric element having a piezoelectric property through thesubstrate. As a result, the piezoelectric element takes a charge. Thecharge is detected by a transparent electrode constituted of a pair oftransparent conductor films. That is, the transparent electrode detectsthe charge generated in the piezoelectric element and outputs thedetection signal.

In a conventional resistance-film piezoelectric sensor, there is a spacebetween transparent conductor films, i.e., transparent conductor filmsare disposed at a predetermined distance from each other. When one ofthe pair the transparent substrates is subjected to outside pressure,the transparent conductor films are deformed to come into contact witheach other, thereby detecting the outside pressure. This has raisedproblems with a flaw occurring due to contact of the transparentconductor films and with durability of the transparent conductor films.

Conversely, the piezoelectric sensor of the present invention has thepair of transparent conductor films opposed to each other with thepiezoelectric element interposed therebetween. When one of the pair ofthe transparent substrates is subjected to outside pressure, thepiezoelectric element takes a charge. The charge is detected by thetransparent conductor films, so that the outside pressure is detected.Therefore, the transparent conductor films do not need to come intocontact with each other, so that a flaw can be prevented from occurringdue to contact. Moreover, the transparent conductor films do not need tobe deformed, so that a piezoelectric sensor having an excellentdurability as compared with the conventional arrangements can beprovided.

It is preferable to arrange the piezoelectric sensor of the presentinvention so that the piezoelectric element is made of aluminum nitrideor zinc oxide.

Among piezoelectric thin films, the piezoelectric thin film made ofaluminum nitride or zinc oxide has a relatively high mechanicalstrength, thereby making it possible to provide a piezoelectric sensorhaving an excellent durability.

It is preferable to arrange the piezoelectric sensor of the presentinvention so that the piezoelectric element has a thickness of 1 μm to10 μm.

When a piezoelectric element is too thin, it is impossible to maintainan insulative property between the transparent conductor films, so thatinsulation failure tends to occur. Further, when a piezoelectric elementis too thick, it takes long time to form the element. However, apiezoelectric element with a thickness of 1 μm to 10 μm can maintain aninsulative property between the transparent conductor films and does nottake so long time to form the element.

The piezoelectric sensor according to the present invention may bearranged so that a further transparent conductor film layer is formed onone of the pair of transparent substrates so as to be positioned in aside opposite to the transparent conductor film layer.

With the foregoing arrangement, a further transparent conductor filmlayer is formed on one of the pair of transparent substrates. That is,one of the transparent substrates is interposed between the transparentconductor films.

For example, in case where one of a pair of transparent conductor filmsopposed to each other with a piezoelectric element therebetween has azero potential at all times and the other detects a charge generated inthe piezoelectric element, when an external unnecessary noise isgenerated, the noise may be detected by the transparent conductor filmwhich detects the charge. As a result, the piezoelectric sensor maymalfunction.

However, the piezoelectric sensor of the present invention is arrangedso that a further transparent conductor film layer is formed on thetransparent substrate so as to be positioned in a side opposite to thetransparent conductor film layer for detecting a charge in thepiezoelectric element. Therefore, even when an external noise isgenerated, the noise is detected by the transparent conductor film layerformed on the opposite side.

Therefore, with the piezoelectric sensor of the present invention, anexternal noise does not reach the transparent conductor film layer fordetecting a charge in the piezoelectric element. This securely preventsan external noise from being detected as a charge in the piezoelectricelement, thereby preventing malfunctioning.

In order to solve the foregoing problems, the method according to thepresent invention for producing the piezoelectric sensor includes thesteps of: forming transparent conductor film layers respectively on apair of transparent substrates; forming a transparent piezoelectricelement having a piezoelectric property so as to cover one of thetransparent conductor film layers which has been formed on one of thepair of transparent substrates; and bonding the piezoelectric element tothe other of the transparent conductor film layers which has been formedon the other transparent substrate not provided with the piezoelectricelement.

With the foregoing arrangement, a piezoelectric sensor which hastransparent conductor films opposed to each other with a piezoelectricelement having a piezoelectric property interposed therebetween isproduced. The piezoelectric sensor has a pair of transparent conductorfilms opposed to each other with a piezoelectric element interposedtherebetween. Further, when one of the pair the transparent substratesis subjected to outside pressure, the piezoelectric element takes acharge. The charge is detected by the transparent conductor films, sothat the outside pressure is detected.

Therefore, the transparent conductor films do not need to come intocontact with each other, so that a piezoelectric sensor can be providedwhich makes it possible to prevent a flaw form occurring due to contact.Moreover, the transparent conductor films do not need to be deformed, sothat a piezoelectric sensor having an excellent durability as comparedwith the conventional arrangements can be provided.

In order to solve the foregoing problems, an input device according tothe present invention is arranged so as to include a plurality ofpiezoelectric sensors each of which is the aforementioned piezoelectricsensor according to the present invention.

That is, the input device is a transparent input device including aplurality of the piezoelectric sensors of the present invention as akeypad.

As described above, since the piezoelectric sensor of the presentinvention, unlike the conventional arrangements, is arranged so that thetransparent conductor film layers do not come into contact with eachother, the piezoelectric sensor makes it possible to prevent a flaw dueto contact and has an excellent durability. Therefore, the input deviceincluding the piezoelectric sensors of the present invention can be usedas an input device having the same effect. Further, outside pressure isdetected by the piezoelectric element, so that a position subjected topressure can be detected with a simple structure.

Further, as a result of various studies, the inventors has completed aninexpensive piezoelectric sensor having a simple structure by depositinga piezoelectric material having no Curie point in a monocrystalline formwhile controlling a dipole orientation degree of the piezoelectricmaterial so as to form a thin film on a metal diaphragm.

That is, the present invention provides a piezoelectric sensor includinga thin metal diaphragm on which a piezoelectric element having no Curiepoint is deposited in a monocrystalline manner so as to form a thinfilm, the diaphragm being mounted on an internal-combustion cylinder andpress-fitted into an opening end of an axial hollow provided in a mainmetal body, the axial hollow having a detection opening positioned inthe cylinder.

In order to solve the foregoing problems, the piezoelectric sensor ofthe present invention is a piezoelectric sensor having pressuredetection means which includes: pressure transmission means fortransmitting pressure; and a piezoelectric element for receiving thepressure so as to convert the pressure into an electrical signal, thepressure being transmitted from the pressure transmission means, thepiezoelectric sensor being characterized in that the piezoelectricelement is made of a piezoelectric material having no Curie point andhas a dipole orientation degree of not less than 75%.

The “piezoelectric material having no Curie point” is a material whichhas a piezoelectric property and does not lose the piezoelectricproperty until a crystal of the material is melted or sublimated, i.e.,a material whose polarity is not inverted along with a temperature rise.Specifically, a substance having a wurtzite structure is an example. Acrystal of the substance having a wurtzite structure has no symmetricproperty and therefore has a piezoelectric property. Moreover, thesubstance is different from a ferroelectric substance such as leadzirconate titanate, has no Curie point, and does not lose apiezoelectric property until the crystal is melted or sublimated.Therefore, since a piezoelectric element made of the piezoelectricmaterial has an excellent durability and does not deteriorate in apiezoelectric property even at high temperatures, the piezoelectricelement does not lose its function as a piezoelectric element even incase of being exposed to a high temperature of nearly 500° C. as in acylinder of an engine. This eliminates the need for providing thepiezoelectric element with cooling means. Moreover, this eliminates theneed for taking a temperature environment into consideration, so that itis no longer necessary to install the piezoelectric element in alow-temperature place, thereby making it possible to simplify astructure. Therefore, a piezoelectric sensor having heat resistance anda simple structure can be provided at low cost.

Further, a “dipole orientation degree” is defined as a percentage atwhich crystalline columns constituting an electric dipole and having thesame polarity (positive or negative) occupy a surface of a thin film. Ifcrystalline columns are oriented in completely random directions, apiezoelectric property of one crystalline column cancels out apiezoelectric property of another crystalline column, so that a thinfilm as a whole loses a piezoelectric property. That is, when thepiezoelectric element has a dipole orientation degree of not more than75%, an apparent piezoelectric constant becomes not more than one halfof that when a dipole orientation degree is 100%, so that apiezoelectric property of the piezoelectric element deteriorates,thereby making it impossible to detect stress and pressuresatisfactorily. When the piezoelectric element is formed so as to have adipole orientation degree of not less than 75%, the foregoing problemdoes not occur, so that the piezoelectric element retains apiezoelectric property. Therefore, the piezoelectric sensor having heatresistance and a simple structure can retain a good piezoelectricproperty.

Note that, a material having no Curie point, e.g., a material having awurtzite structure is different from a ferroelectric substance, and adipole direction of the material cannot be controlled by an externalelectric field after a crystal of the material has been formed, so thatit is impossible to control a dipole direction of each of thecrystalline columns after a thin film of the crystalline column has beenformed. Therefore, it is necessary to ensure that the thin film has agood piezoelectric property by controlling a dipole orientation of thecrystal when the thin film of the crystal is formed.

Note that, such a piezoelectric element may be applied to thepiezoelectric element of the transparent piezoelectric sensor.

Further, in order to solve the foregoing problem, the piezoelectricsensor of the present invention, in addition to the foregoingarrangement, is arranged so that the piezoelectric element is made ofaluminum nitride (AlN) or zinc oxide (ZnO).

Since AlN and ZnO are substances each having a wurtzite structure, and acrystal of AlN and a crystal of ZnO have no symmetric property, AlN andZnO naturally have a piezoelectric property. Moreover, unlike aferroelectric substance, AlN and ZnO have no Curie point and are notinverted in polarity even at high temperatures, so that they do not losea piezoelectric property until the crystals are melted or sublimated.

For example, AlN has a sublimation temperature of 2000° C., so that AlNdoes not lose a piezoelectric property until 2000° C. Since a combustiontemperature inside an engine cylinder is about 500° C., a piezoelectricelement made of AlN does not need to be provided with cooling means toretain a piezoelectric property. Therefore, the piezoelectric elementmade of the piezoelectric material has an excellent heat resistance, anda piezoelectric property of the piezoelectric element does notdeteriorate. Further, the piezoelectric element has an excellentprocessabilty and is suitable for lamination.

Such a piezoelectric element having heat resistance does not need to beprovided with cooling means and does not need to be installed in alow-temperature place, so that a piezoelectric sensor having a simplestructure and heat resistance can be provided at low cost.

Further, in order to solve the foregoing problems, the piezoelectricsensor of the present invention, in addition to the foregoingarrangement, is arranged so that the piezoelectric element is formed byphysical vapor deposition process.

The “physical vapor deposition process” is a process in which asubstance is evaporated by a physical process and concentrated on a filmmaterial so as to form a thin film. Examples are a sputtering processand a vacuum deposition process. With this method, a needle-shapedcrystal of the piezoelectric material grows to a frost column crystal,so that a monocrystalline thin film of the piezoelectric material can beformed.

Note that, when the crystalline column is subjected to a stress, bothends of the crystalline column respectively take positive and negativecharges to form an electric dipole, and it depends on a direction of thedipole of the crystalline column which ends takes the positive charge.Therefore, in order to increase a dipole orientation degree and ensurethat a thin film of a piezoelectric element has a good piezoelectricproperty, it is necessary to control a dipole orientation of a crystalwhen a thin film of the crystal is formed. For example, it is necessaryto arrange a c-axis orientation of the crystal by setting an optimumsubstrate temperature, an optimum distance between substrate targets,and an optimum gas pressure when a piezoelectric element is formed by aphysical vapor deposition.

Further, in order to solve the foregoing problems, the piezoelectricsensor of the present invention, in addition to the foregoingarrangement, is arranged so that the piezoelectric element has athickness of 0.1 μm or more to 100 μm or less.

This is because the piezoelectric element with a thickness of less than0.1 μm makes it difficult to serially form films and tends toshort-circuit electrodes when one of the electrodes is disposed on topof the other, and the piezoelectric element with a thickness of morethan 100 μm takes long time to form. Therefore, when the piezoelectricelement having a thickness within the limits makes it possible toproduce in a short period of time a piezoelectric sensor which candetect stress and pressure satisfactorily.

Further, in order to solve the foregoing problems, the piezoelectricsensor of the present invention, in addition to the foregoingarrangement, is arranged so that the pressure transmission means isconstituted of a metal diaphragm, and the pressure detection means isformed by providing a piezoelectric element on a surface of the metaldiaphragm.

Here, a “diaphragm” means a film-like body which is deformed in responseto pressure. Further, the phrase that “the metal diaphragm and apiezoelectric element formed on a surface thereof” means not only thatthe piezoelectric element is formed directly on the diaphragm, but alsothat the piezoelectric element is formed on the diaphragm with a baselayer or an electrode layer, which smoothes a surface of the diaphragminterposed therebetween.

With the foregoing arrangement, the pressure detection means is made ofa thin metal diaphragm and the metal diaphragm has a piezoelectricelement on a surface thereof, thereby providing thin pressure detectionmeans having a simple structure. Further, a complex structure such as apressure transmission bar is omitted. Moreover, the metal diaphragm usedin case of this arrangement is different from the conventionalarrangements, is used only for transmitting pressure to thepiezoelectric element, and is subjected not to a deflective strain butto a compressive strain. Therefore, the metal diaphragm is strained onlyslightly due to the pressure, and the piezoelectric element formedthereon is also only slightly strained, thereby eliminating the need fora structure for preventing the piezoelectric element from being damageddue to an excessive strain. This makes it possible to simplify astructure of the piezoelectric element.

Note that, a thin ceramic sintered body is used for the pressuredetection means; however, since such a thin ceramic sintered body ispoor at withstanding a physical shock and a heat shock, it is preferableto use a metal.

Further, in order to solve the foregoing problems, the piezoelectricsensor of the present invention, in addition to the foregoingarrangement, is arranged so as to include a main metal body for mountingthe pressure detection means on an internal-combustion cylinder, themain metal body having an axial hollow for connecting an inside of thecylinder with an outside of the cylinder, the pressure detection meansbeing provided in the axial hollow.

The piezoelectric sensor of the present invention has an excellent heatresistance and can be more effectively used particularly for measuringinternal pressure of an internal-combustion cylinder. Aninternal-combustion engine has a combustion temperature of about 500°C., and a piezoelectric element exposed in the internal-combustionengine reaches a temperature of about 400° C. Accordingly, a complexarrangement was needed in which the piezoelectric element was cooled orwas installed in a low-temperature place remote from theinternal-combustion engine. However, the piezoelectric sensor of thepresent invention eliminates the need for taking a temperatureenvironment into consideration and therefore achieves a simplestructure.

Further, as a result of various studies of a method of forming a thinfilm of a piezoelectric material having no Curie point, the inventorshave found that the foregoing objects can be achieved by depositing apiezoelectric ceramic in a monocrystalline manner while controlling apolarity of the piezoelectric ceramic so as to form a thin film on aninsulative substrate (made of an oxide, carbide, nitride, or borideceramic sintered body or quartz glass) or a conductive substrate (madeof a heat-resistant metal material equivalent to Inconel or SUS630).

That is, a thin-film piezoelectric sensor according to the presentinvention is a high-temperature thin-film piezoelectric sensor includingan insulative substrate (made of an oxide, carbide, nitride, or boridesintered body or quartz glass) or a conductive substrate (made of aheat-resistant metal material equivalent to Inconel or SUS630) on whicha piezoelectric ceramic having no Curie point is deposited in amonocrystalline manner so as to form a thin film. The high-temperaturethin-film piezoelectric sensor can be used for various types ofthin-film piezoelectric sensor such as a compression type, a cantilevertype, a diaphragm type, and a shear type.

The piezoelectric sensor of the present invention is arranged so thatthe pressure transmission means is a substrate, and a first conductorfilm layer, the piezoelectric element, and a second conductor film layerare laminated on a surface of the pressure transmission means in thisorder.

The thin-film piezoelectric sensor of the present invention has thesubstrate on which the first conductor film layer, the piezoelectricelement, and the second conductor film layer are laminated andintegrated, so that the thin-film piezoelectric sensor has a simple,small structure. Further, the first conductor film layer functions as abase layer for improving a dipole orientation degree of thepiezoelectric element, thereby improving a piezoelectric property.

Further, in order to solve the foregoing problems, the thin-filmpiezoelectric sensor of the present invention, in addition to theforegoing arrangement, is arranged so that the substrate is aninsulative substrate made of an oxide, carbide, nitride, or borideceramic sintered body or quartz glass. With this arrangement, theceramic material has an excellent heat resistance, can be producedeasily at low cost, has high hardness, and has an accurate property, sothat a high-performance thin-film piezoelectric sensor having highproductivity can be obtained.

Further, the substrate may be a conductive substrate made of aheat-resistant metal material. With this arrangement, the substrate canbe used as a substitute for a lead wire for extracting a signal from thefirst conductor film layer, and the substrate can be formed into variousshapes by normal machining.

Further, in order to solve the foregoing problems, the piezoelectricsensor of the present invention is arranged so that the first conductorfilm layer has a surface, being in contact with the piezoelectricsensor, which is coated with a metal contained in the piezoelectricsensor.

Here, the “metal contained in the piezoelectric sensor” is a metalserving as a principal component contained in a material for thepiezoelectric element, e.g., aluminum when the piezoelectric element ismade of aluminum nitride, and zinc when the piezoelectric element ismade of zinc oxide. Further, only a surface of the first conductor filmlayer in contact with the piezoelectric element may be coated with ametal contained in the piezoelectric element, but a whole of the firstconductor film layer may be made of a metal contained in thepiezoelectric element. That is, for example, when a material for thepiezoelectric element is aluminum nitride, a material for the firstconductor film layer may be aluminum; when a material for thepiezoelectric element is zinc oxide, a material for the first conductorfilm layer may be zinc.

This causes a dipole orientation degree of the piezoelectric element torise to 75% and enables the piezoelectric element to retain apiezoelectric property, so that the thin-film piezoelectric sensor candetect stress satisfactorily.

Further, in order to solve the foregoing problems, the piezoelectricsensor of the present invention, in addition to the foregoingarrangement, is arranged so that the second conductor film layer isdivided into two or more.

With this arrangement, when different positions in the thin-filmpiezoelectric sensor are subjected to different stresses or pressures,different stresses are generated by different electrodes, so thatdifferent charges and voltages are generated on the differentelectrodes. In case of a cantilever-type and diaphragm-typepiezoelectric sensor, there is a situation where it is effective interms of sensitivity to detect a stress difference in the substrateprovided with a piezoelectric thin film (i.e., a difference between theelectrodes). Particularly, in case of detecting a shear stress, not abending stress, in the cantilever-type piezoelectric sensor, it becomespossible to detect a difference with a hardware device, therebyachieving high-sensitivity detection without restrictions placed on adynamic range of an amplifier.

For a fuller understanding of the nature and advantages of theinvention, reference should be made to the ensuing detailed descriptiontaken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a cross-sectional view of a piezoelectric sensor according toone embodiment of the present invention.

FIG. 2 is a perspective view of a transparent input device using thepiezoelectric sensor according to one embodiment of the presentinvention.

FIG. 3 is a perspective view of a transparent input device havingconcentric transparent conductor film layers using the piezoelectricsensor according to one embodiment of the present invention.

FIG. 4 is a response curve graph of a piezoelectric sensor of Example 1.

FIG. 5 is a diagram illustrating the piezoelectric sensor of Example 1.

FIG. 6 is a longitudinal sectional view of the piezoelectric sensoraccording to one embodiment of the present invention.

FIG. 7 is a cross-sectional view of a diaphragm of the piezoelectricsensor according to one embodiment of the present invention.

FIG. 8 is a cross-sectional view of a laminated substrate of thepiezoelectric sensor according to one embodiment of the presentinvention.

FIG. 9 is a cross-sectional view of a laminated substrate of a thin-filmpiezoelectric sensor, according to another embodiment of the presentinvention, having a plurality of divided upper electrodes formedthereon.

FIG. 10 is a graph showing a result of an oscillation detectionmeasurement using a thin-film piezoelectric sensor according to oneembodiment of the present invention.

BEST MODE FOR CARRYING OUT THE INVENTION First Embodiment

One embodiment of the present invention is described below withreference to FIGS. 1 to 4. Note that, this is not for limitation of thepresent invention.

A piezoelectric sensor according to the present invention is arranged sothat a laminated electrode constituted of transparent conductor filmlayers opposite to each other and a transparent pressure-sensitive layer(piezoelectric element) having a piezoelectric property interposedtherebetween are laminated and integrated on a pair of transparentsubstrates opposite to each other. As a whole, the piezoelectric sensoris a transparent piezoelectric sensor.

First, an arrangement of the piezoelectric sensor according to thepresent embodiment will be described.

FIG. 1 is a cross-sectional view illustrating a structure of thepiezoelectric sensor according to the present embodiment. As illustratedin FIG. 1, the piezoelectric sensor of the present embodiment isconstituted of: a transparent pressure-sensitive layer 1; transparentconductor film layers 2, 2′, and 2″; transparent insulative substrates 3and 3′; an electric circuit 4; and detection means 5.

The transparent pressure-sensitive layer 1 is made of a piezoelectricmaterial whose surface is charged by pressure. A material (piezoelectricmaterial) for the transparent pressure-sensitive layer 1 is notparticularly limited provided that it has a piezoelectric property andcan insulate the transparent conductor film layers 2 and 2′ from eachother. Examples are a piezoelectric crystalline material such ascrystal, LiNbO₃, LiTaO₃; a PbZrO₃—PbTiO₃ material generated out of asolid solution of PbZrO₃ and PbTiO₃ (i.e., a piezoelectric ceramicmaterial such as a PZT material); a piezoelectric thin film materialsuch as aluminum nitride and zinc oxide; and a polymeric piezoelectricmaterial such as vinylidene polyfluoride and vinyl polyfluoride. Amongthem, the piezoelectric thin film material is preferable; that is,aluminum nitride and zinc oxide are more preferable. Aluminum nitrideand zinc oxide have a relatively high mechanical strength and thereforeare particularly suitable for the transparent pressure-sensitive layer1.

A thickness of the transparent pressure-sensitive layer 1 is notparticularly limited provided that the transparent pressure-sensitivelayer 1 can insulate the transparent conductor film layers 2 and 2′ tobe described later from each other, and can be suitably changed ifnecessary. However, it is preferable that when made of aluminum nitrideor zinc oxide, the transparent pressure-sensitive layer 1 have athickness of 1 μm to 10 μm. This is because the transparentpressure-sensitive layer 1 with a thickness of less than 1 μm may causean insulation failure, and the transparent pressure-sensitive layer 1with a thickness of more than 10 μm takes longer time to form.

The transparent conductor film layers 2 and 2′ detect a charge generatedby pressurizing the transparent pressure-sensitive layer 1. Further, thetransparent conductor film layer 2″, as described later, prevents anexternal noise from being detected.

A material for the transparent conductor film layers 2, 2′, and 2″ isnot particularly limited, but is for example a metal such as platinum(Pt), chrome (Cr), gold (Au), cupper (Cu), silver (Ag), aluminum (Al),and tantalum (Ta) as well as an alloy such as silver-nickel (Ag—Ni).

Further, the transparent conductor film layers 2, 2′, and 2″ do not needto be made of the same material, but can be made of different materialsin accordance with their compatibilities with the transparentpressure-sensitive layer 1 and applications thereof.

A material for the transparent substrates 3 and 3′ is for example aninorganic material (e.g., glass) and a resin film (e.g., polyimide,polyethylene terephthalate, polycarbonate, and polyphenylene sulfide).Note that, the transparent substrates 3 and 3′ do not need to be made ofthe same material, but can be made of different materials in accordancewith their compatibilities with the transparent conductor film layers 2and 2′ and applications thereof.

Thus, the piezoelectric sensor of the present embodiment is arranged sothat: (i) the transparent pressure-sensitive layer 1 having apiezoelectric property and (ii) the pair of transparent conductor filmlayers 2 and 2′ opposite to each other with the transparentpressure-sensitive layer 1 interposed therebetween are provided betweenthe pair of transparent insulative substrates 3 and 3′ opposite to eachother, and the transparent conductor film layer 2″ is provided on thetransparent insulative substrate 3 so as to be positioned in a sideopposite to the transparent conductor film layer 2. Note that, it may beso arranged that the piezoelectric sensor of FIG. 1 is not provided witha transparent conductor film layer 2″. However, in order to securelyprevent an external unnecessary noise from interfering, it is preferablethat the transparent conductor film layer 2″ be provided.

Further, the piezoelectric sensor of FIG. 1 is arranged so that thetransparent conductor film layers 2 and 2′ are electrically connected bythe electric circuit 4 and connected through the electric circuit 4 tothe detection means 5. Further, the transparent conductor film layers 2′and 2″ maintain a constant potential, i.e., a zero potential at alltimes. Further, the transparent conductor film layer 2 is connectedthrough the electric circuit 4 to the detection means 5 and detects acharge generated in the transparent pressure-sensitive layer 1.

The detection means 5 is for example a display device (e.g., a voltmeterusing a charge sensitive amplifier and a voltage amplifier) and a devicefor taking in an output voltage of those amplifiers through an A/D(audio-to-digital) converter into a computer.

In the following, operation of the piezoelectric sensor of FIG. 1 underpressure will be described. Note that, in the following description,outside pressure is applied from the side of the transparent insulativesubstrate 3′.

When the transparent insulative substrate 3′ is pressed by an object,the transparent insulative substrate 3′ applies pressure to thepiezoelectric sensor of FIG. 1 according to the foregoing arrangement.The pressure causes the transparent pressure-sensitive layer 1 servingas a piezoelectric body to take a charge. As a result, the transparentconductor film layer 2 takes the same potential as that of the chargegenerated in the transparent pressure-sensitive layer 1. Further, thetransparent conductor film layers 2′ and 2″ maintain a zero potential atall times regardless of whether the transparent pressure-sensitive layer1 takes a charge or not. That is, there occurs a potential differencebetween the transparent conductor film layer 2 and the transparentconductor film layers 2′ and 2″ electrically connected to each other bya conductor. The potential difference is detected by the detection means5 connected through the electric circuit 4.

Further, the piezoelectric sensor of FIG. 1 is provided with thetransparent conductor film layer 2″. Therefore, even when an externalunnecessary noise is generated, the noise is shielded by the transparentconductor film layers 2′ and 2″. That is, the transparent conductor filmlayer 2 is shielded from an outside by the transparent conductor filmlayer 2″. This makes it possible to detect only a charge generated inthe transparent pressure-sensitive layer 1 without detecting an externalnoise.

Thus, the piezoelectric sensor of the present embodiment is arranged sothat: when the transparent insulative substrate 3′ is subjected tooutside pressure, the transparent pressure-sensitive layer 1 takes acharge, and the transparent conductor film layer 2 detects the charge.Therefore, the transparent conductor film layers 2 and 2″ do not need tocome into contact with each other. This prevents a flaw from occurringdue to contact. Moreover, the transparent conductor film layers 2 and 2″do not need to be deformed, thereby ensuring an excellent durability ascompared with the conventional arrangements. Further, the piezoelectricsensor is provided with the transparent conductor film layer 2″, therebysecurely preventing an external unnecessary noise from interfering.

The piezoelectric sensor illustrated in FIG. 1 can be produced forexample by the following steps (1) to (3) of:

(1) forming the transparent conductor film layers 2 and 2′ respectivelyon the transparent insulative substrates 3 and 3′;

(2) forming the transparent pressure-sensitive layer 1 so as to coverthe transparent conductor film layer 2 formed on one (e.g., thetransparent insulative substrate 3) of the transparent insulativesubstrates 3 and 3′; and

(3) bonding the transparent pressure-sensitive layer 1 to thetransparent conductor film layer 2′ formed on the transparent insulativesubstrate 3′ so as to be positioned in a side opposite to thetransparent pressure-sensitive layer 1 formed in step (2).

Step (1) may further includes the step of forming the transparentconductor film layer 2″ in order to securely prevent an externalunnecessary noise from interfering.

Further, in step (1), the transparent conductor film layers 2, 2′, and2″ can be formed by any one of the publicly known processes such as aprinting process, a thin film treatment process, a sputtering process,an evaporation process, an ion plating process, and a bonding process,and a forming process thereof is not particularly limited.

The transparent pressure-sensitive layer 1 formed in the step (2) can beformed by any one of the publicly known processes such as a sputteringprocess, the ion plating process, a CVD (chemical vapor deposition)process, and a PVD (physical vapor deposition) process, and a formingprocess thereof is not particularly limited.

As described in an example below, the bonding in step (3) is carried outfor example with a cyanoacrylate-based binder, but a bonding process isnot particularly limited.

Thus, a piezoelectric sensor having an excellent durability and ananti-noise property as compared with the conventional arrangements canbe produced in the simple steps (1) to (3). Note that, an input deviceto be described later having a plurality of the piezoelectric sensors ofthe present invention can be produced in the same steps.

In the following, one example of applications of the piezoelectricsensor of the present invention will be described. Whereas onepiezoelectric sensor of the present invention described above functionsas a touch key, a plurality of the piezoelectric sensors can be used asa keypad (input device).

FIG. 2 illustrates a back side (a side of a substrate which side isopposite to the side subjected to outside pressure) of one example ofthe embodiment. That is, FIG. 2 illustrates the piezoelectric sensor ofFIG. 1 seen from the side of the transparent insulative substrate 3.Note that, in FIG. 2, explanations of the transparent pressure-sensitivelayer 1, the transparent conductor film layer 2″, and the transparentsubstrates 3 and 3′ are omitted. Further, in the following, a surface(panel surface) side (the side of the transparent insulative substrate3′ in FIG. 1) which is subjected to outside pressure is called an “upperpart”, and an opposite side thereof is called a “lower part”. Further,based on this, the transparent conductor film layers 2, 2′, and 2″ arerespectively called an upper transparent conductor film layer 2′, anintermediate transparent conductor film layer 2, and a lower transparentconductor film layer 2″ in an order from top to bottom.

A touch panel illustrated in FIG. 2 is formed as follows. First, a lowertransparent conductor film layer 2″ is formed on an entire surface of alower transparent insulative substrate 3. Next, an intermediatetransparent conductor film layer 2 is formed on the lower transparentinsulative substrate 3 so as to be positioned in a side opposite to thelower transparent conductor film layer 2″. Then, a transparentpressure-sensitive layer 1, an upper transparent conductor film later2′, and the upper transparent insulative substrate 3′ are formedentirely on an upper surface of the intermediate transparent conductorfilm layer 2 in this order. The upper transparent conductor film layer2′ and the lower transparent conductor film layer 2″ are electricallyconnected by a conductive body such as a lead wire and maintain a zeropotential at all times. The intermediate transparent conductor filmlayer 2 is divided into as many keys as needed for a keypad. Each of theintermediate transparent conductor film layers 2 so divided is providedwith an output terminal 6 for external connection. Note that, the uppertransparent insulative substrate 3′ may be divided for the keys or maybe integrated as a single insulative substrate. When the uppertransparent insulative substrate 3′ is integrated, a cross talk mayoccur. However, since each of the keys has a different output due to apressure distribution of the key being pressed, it is possible todistinguish an output of the key according to an output value and delaytime.

Note that, the intermediate transparent conductor film layer 2 may takethe shape of a matrix as illustrated in FIG. 2 or a concentric circle asillustrated in FIG. 3. When the intermediate transparent conductor filmlayer 2 takes the shape of a matrix, the intermediate transparentconductor film layer 2 can be used as a normal keyboard. On the otherhand, when the intermediate transparent conductor film layer 2 takes theshape of a concentric circle, it is possible to easily obtain contactinformation in accordance with a distance from the center of a roundpanel. Therefore, for example, when the intermediate transparentconductor film layer 2 is used for a batter head of an electric drum, itbecomes possible to identify a location in contact with a stick, therebymaking it possible to change a voice of a generated sound.

Such an input device of the present invention is provided with apiezoelectric sensor of the present invention and therefore, unlike theconventional arrangements, prevents transparent conductor film layersfrom coming into contact with each other. This makes it possible toprevent a flaw due to contact of transparent conductor film layers witheach other. Further, since the transparent conductor films do not needto be deformed, the input device has an excellent durability.

Note that, the piezoelectric sensor of the present invention may be as awhole a transparent piezoelectric sensor in which a pair of transparentconductor film layers opposite to each other and a pressure-sensitivelayer having a piezoelectric property interposed therebetween arelaminated and integrated onto a transparent substrate.

With this arrangement, the transparent conductor films do not need tocome into contact with each other, so that it is possible to prevent aflaw from occurring due to contact. Moreover, the transparent conductorfilms do not need to be deformed, so that a piezoelectric sensor havingan excellent durability as compared with the conventional arrangementscan be provided.

Further, the piezoelectric sensor of the present invention may bearranged so that: transparent conductor film layers are formed on bothends of a transparent pressure-sensitive layer having a piezoelectricproperty, and they are formed between a pair of transparent substrates.That is, the pair of transparent substrates may be in contact with (i)the transparent pressure-sensitive layer and (ii) the transparentconductor films formed on both ends of the transparentpressure-sensitive layer.

Further, the piezoelectric sensor of the present invention may be apiezoelectric sensor including a pair of substrates opposite to eachother respectively provided with transparent conductor film layers,wherein a transparent pressure-sensitive layer is formed between thetransparent conductor film layers.

With this arrangement, when one of the pair of the transparentsubstrates is subjected to pressure, the pressure acts on thetransparent pressure-sensitive layer, thereby causing the transparentpressure-sensitive layer to take a charge. The charge is detected by thetransparent conductor film layers provided on both ends of thetransparent pressure-sensitive layer, so that outside pressure can bedetected. Therefore, the transparent conductor film layers do not needto come into contact with each other. This makes it possible to preventa flaw from occurring due to contact. Moreover, the transparentconductor films do not need to be deformed, so that a piezoelectricsensor having an excellent durability as compared with the conventionalarrangements can be provided.

A method according to the present invention for producing thepiezoelectric sensor may include the step of forming apressure-sensitive layer having a piezoelectric property on a pair ofsubstrates provided with transparent conductor film layers so that thetransparent conductor films are adjacent to each other with thepressure-sensitive layer interposed therebetween.

With the foregoing arrangement, a piezoelectric sensor having a pair oftransparent conductor films opposite to each other with a transparentpressure-sensitive layer having a piezoelectric property interposedtherebetween can be provided. In other words, a piezoelectric sensorhaving a pair of transparent conductor films opposite to each other witha pressure-sensitive layer interposed therebetween is produced. Thepiezoelectric sensor is arranged so that the transparent conductor filmlayers do not need to come into contact with each other, so that it ispossible to prevent a flaw from occurring due to contact. Moreover, thetransparent conductor film layers do not need to be deformed, so that apiezoelectric sensor having an excellent durability as compared with theconventional arrangements can be provided.

Example 1

An example of the piezoelectric sensor according to the First Embodimentwill be described below.

A glass substrate with ITO (i.e., a glass substrate whose one surface iscoated with ITO) was used as a transparent substrate and a transparentconductor film layer. The glass substrate with ITO had a thickness of 1mm. Next, a thin film of aluminum nitride with a thickness of 1 μmserving as a transparent pressure-sensitive layer was formed on ITO ofthe glass substrate with ITO by a sputtering process.

Then, another glass substrate with ITO was prepared, and its ITO layerwas bonded to the transparent pressure-sensitive layer of aluminumnitride with a cyanoacrylate-based binder.

That is, the piezoelectric sensor of the present invention was made of aglass substrate (transparent insulative substrate layer), ITO (atransparent conductor film layer), aluminum nitride (a piezoelectricelement), ITO (a transparent conductor film layer), a glass substrate(transparent insulative substrate layer), that were laminated as layersin this order.

Here, a part serving as a transparent sensor having transparentelectrodes opposite to each other (i.e., three layers made of a pair ofITO layers and aluminum nitride interposed therebetween) was 15 mm bothin length and width.

Further, each of the ITO layers (transparent conductor film layers) wasprovided with a lead wire and connected through a charge sensitiveamplifier to a synchroscope, thereby producing a piezoelectric sensor.FIG. 5 shows the piezoelectric sensor so produced which is placed on asheet. As shown in FIG. 5, characters on the sheet are seen through asensor section of the piezoelectric sensor so produced. That is, thepiezoelectric sensor is undoubtedly transparent.

A rectangular pressure with a frequency of 1 Hz was applied to thepiezoelectric sensor with an electric pressurizer to see a response ofthe sensor. As a result, as shown in FIG. 4, a charge which wasgenerated when a pressure of about 40 Pa was applied to thepiezoelectric sensor was different from a charge which was generatedwhen pressure was released.

Second Embodiment

One embodiment of the present invention will be described below withreference to FIGS. 6 and 7.

A metal diaphragm (pressure transmission means) having the piezoelectricthin film layer (piezoelectric element) is press-fitted into an openingend of a hole, leading into an internal-combustion cylinder, of a mainmetal body, and constitutes a piezoelectric sensor for detecting aphenomenon in the cylinder.

FIG. 6 is a longitudinal sectional view of the piezoelectric sensor ofthe present embodiment for measuring internal pressure of aninternal-combustion cylinder.

The piezoelectric sensor, constituted of a signal transmission section15, pressure detection means 23, and a cap 24, detects pressure exertedfrom a space on the pressure detection means 23 side so as to output anelectrical signal.

The pressure detection means 23, provided at a detection opening,receives pressure and converts the pressure into an electrical signal.The pressure detection means 23 will be described in detail later.

The signal transmission section 15, constituted of a main metal body 21,a signal output bar 28, and an electrically insulative column 27,transmits to a signal transportation cable an electrical signaloutputted by the pressure detection means 23.

The main metal body 21 has a bolt structure, is constituted of anupper-end portion 21 c, an upper portion 21 d, a lower portion 21 a, anda lower-end portion 21 b, and is provided with an axial hollow 26passing from an upper end through a lower end in an inside thereof.

The upper-end portion 21 c, having an upper-end male screw 22 c in aperimeter thereof, can be screwed to the signal transportation cable(not shown). The upper portion 21 d, having a larger diameter than anyother portion, has a corner in a perimeter thereof. The upper portion 21d plays a role as a hexagonal part which fits in a tightening tool suchas a wrench in case of screwing the main metal body 21 to anothermember. The lower portion 21 a, having a lower-end male screw 22 a in aperimeter thereof, can be screwed to a cylinder block of a cylinder formeasuring pressure. The lower-end portion 21 b has a lower-end malescrew 22 b in a perimeter thereof. The lower-end male screw 22 b canscrew a cap 24 for confining the pressure detection means 23 in a lowerend of the main metal body 21.

The axial hollow 26 is an articulated hollow constituted of alarge-diameter hollow 26 a on the lower-end portion side and asmall-diameter hollow 26 b on the upper-end portion side. Theelectrically insulative column 27 is inserted into the large-diameterhollow 26 a. The signal output bar 28, in parallel to the axial hollow26, is provided so as to pass through the center of the electricallyinsulative column 27. The signal output bar 28 passes through thesmall-diameter hollow 26 b, and an upper end of the signal output bar 28is connected to the signal transportation cable (not shown) at theupper-end portion 21 c of the main metal body 21. Provided at an end ofa lower-end portion side of the signal output bar 28 is an electrode 29,which is made of a metal. A lower surface of the electrode 29 isarranged so as to come into contact with the pressure detection means 23at a lower-end portion of the axial hollow 26. A lower end of the axialhollow 26, located in a cylinder, is a detection opening to bepressurized by the cylinder. In the detection opening, the pressuredetection means 23 detects internal pressure of the cylinder. Further,the electrode 29 and the signal output bar 28 are in contact only withthe electrically insulative column 27 in the axial hollow 26, and areelectrically insulated from the main metal body 21.

The cap 24, mounted on the lower end of the main metal body 21, ispressed onto the lower end of the axial hollow 26 so as to cover thepressure detection means 23. The center of the cap 24 is provided withan opening whose diameter is smaller than that of the pressure detectionmeans so that a lower surface of a central portion of the pressuredetection means 23 is exposed even after the pressure detection means 23is pressed onto the lower end of the axial hollow 26 with the cap 24.That is, a peripheral portion of the opening of the cap 24 press-fitsthe pressure detection means 23 into the main metal body 21. The cap 24is screwed to the lower-end male screw 22 b in the lower end of the mainmetal body 21 and keeps press-fitting the pressure detection means 23into the main metal body 21.

In the following, the pressure detection means 23 will be described indetail with reference to FIG. 7.

The pressure detection means 23 is arranged so that: on a metaldiaphragm (pressure transmission means) 10, a base layer 11, apiezoelectric thin film layer (piezoelectric element) 12, and an upperelectrode 13 are laminated in this order.

Each of the films can be formed by a physical vapor deposition (PVD)process (which is a process by which a substance is evaporated by aphysical method and concentrated on a member on which the substance isto be thinned so as to form a thin film). Examples of the PVD processare vacuum deposition processes (e.g., a resistance heating depositionor an electron beam heating deposition process), various sputteringprocesses (e.g., a DC sputtering process, a high-frequency sputteringprocess, an RF plasma support sputtering process, a magnetron sputteringprocess, an ERC sputtering process, and an ion beam sputtering process),various ion plating processes (e.g., a high-frequency ion platingprocess, an active deposition process, and an arc ion plating process),a molecular beam epitaxy process, a laser ablation process, an ioncluster beam deposition process, and an ion beam deposition process.

FIG. 7 is a cross-sectional view of the pressure detection means 23according to the present embodiment. The pressure detection means 23 isarranged so that: on the metal diaphragm 10, the base layer 11, thepiezoelectric thin film layer 12, and the upper electrode 13 arelaminated in this order. When the pressure detection means 23 is mountedon the main body 21, the pressure detection means 23 is mounted on anopening in the lower end of the axial hollow 26 so that the upperelectrode 13 is press-fitted onto the electrode 29.

The metal diaphragm 10 transmits pressure to the piezoelectric thin filmlayer 12 by coming into contact with a space where the pressure ismeasured, and serves as a substrate for supporting the pressuredetection means 23. The metal diaphragm 10 is located in aninternal-combustion cylinder which reaches a high temperature, so thatthe metal diaphragm needs to have heat resistance. It is preferable thatthe metal diaphragm 10 be made for example of a heat-resistant metalmaterial equivalent to Inconel or SUS630. It is desirable that a surfaceon which the piezoelectric thin film layer 12 is formed bemirror-finished by a polishing or chemical method in order to preventthe piezoelectric thin film layer 12 from cracking and peeling andenhance an orientation of a crystal axis of the piezoelectric thin filmlayer 12.

Further, the base layer 11 is a buffer layer between the piezoelectricthin film layer 12 produced thereon and the metal-diaphragm 10. The baselayer 11 orients a polarity of the piezoelectric thin film layer 12,orients the crystal axis, and improves wettability of the piezoelectricthin film layer 12 with respect to the metal diaphragm 10. Further thebase layer 11 is used also as a lower electrode.

The base layer 11 may be made of TiN, MoSi₂, Si₃N₄, Cr, Fe, Mg, Mo, Nb,Ta, Ti, Zn, Zr, W, Pt, Al, Ni, Cu, Pd, Rh, Ir, Ru, Au, or Ag, and may beconstituted of a single layer, or two or more layers made of a pluralityof materials.

The piezoelectric thin film layer 12 is subjected to pressuretransmitted through the metal diaphragm 10 and the base layer 11 andoutputs an electrical signal in accordance with the pressure. That is, apressure under test is applied to the piezoelectric thin film layer 12and converted by the piezoelectric thin film layer 12 into an electricalsignal. It is desirable that the piezoelectric thin film layer 12 bemade of aluminum nitride (AlN) or zinc oxide (ZnO) by a sputteringprocess.

The upper electrode 13, press-fitted onto the electrode 29 in the mainmetal body 21, transmits a charge, generated by an applied pressure,through the electrode 29 and the signal output bar 28 to the signaltransportation cable (not shown). The upper electrode 13 may be made ofthe same materials as the base layer 11, but not necessarily. The upperelectrode 13 may be made of a material appropriately chosen inaccordance with its compatibility with the piezoelectric thin film layer12 and the electrode 29, and may be constituted of a single layer.

In the following, an operation of the piezoelectric sensor will bedescribed. The metal diaphragm 10 receives pressure and transmits thepressure to the piezoelectric thin film layer 12. The piezoelectric thinfilm layer 12 converts the pressure into an electrical signal. Theelectrical signal is transmitted to from the upper electrode 13 throughthe electrode 29 to the signal output bar 28 and further transmittedfrom the upper end of the main metal body 21 to the signaltransportation signal, so that a pressure display section (not shown)displays a pressure under test.

Note that, a material for the piezoelectric thin film layer 12 is notlimited to aluminum nitride (AlN) and zinc oxide (ZnO), but may be anypiezoelectric material having no Curie point. GaN is an example. Acrystal of such a piezoelectric material does not lose a piezoelectricproperty until the crystal is melted or sublimated. A crystal of asubstance having a wurtzite structure has no symmetric property andtherefore has a piezoelectric property. Further, the crystal is not aferroelectric substance and therefore has no Curie point. Therefore, thepiezoelectric sensor made of the piezoelectric material has an excellentdurability, does not deteriorate in a piezoelectric property, and doesnot lose its function as a piezoelectric element even when exposed to ahigh temperature of 500° C. as in a cylinder of an engine. This makes itunnecessary to provide the piezoelectric element with cooling means orinstall the piezoelectric element in a low-temperature place, therebysimplifying the structure of the piezoelectric element.

Further, the piezoelectric sensor of the present invention includes themetal diaphragm 10 and a thin layer such as the piezoelectric thin filmlayer 12 formed thereon.

With this arrangement, the pressure detection means 23 has the metaldiaphragm 10 and a thin layer such as the piezoelectric thin film layer12 formed thereon, so that the pressure detection means 23 becomes thinand small. Moreover, the metal diaphragm used in this arrangement isdifferent from the conventional arrangements, is used only fortransmitting pressure to the piezoelectric element, and is subjected notto a deflection or strain but to a compressive strain. Therefore, themetal diaphragm is strained so slightly due to the pressure, and thepiezoelectric element formed thereon is also so slightly strained, sothat it is not necessary to provide a structure for preventing anexcessive strain from damaging the piezoelectric element. This makes itpossible to achieve an inexpensive piezoelectric element having a simplestructure.

Further, it is desirable that the piezoelectric thin film layer 12 ofthe piezoelectric sensor of the present invention have a thickness of0.1 μm to 100 μm. Further, it is more preferable that the piezoelectricthin film layer 12 have a thickness of 0.5 μm or more to 20 μm or less.It is even more preferable that the piezoelectric thin film layer 12have a thickness of 1 μm or more to 10 μm or less. The piezoelectricthin film layer 12 with a thickness of less than 0.1 μm tends toshort-circuit the base layer 11 and the upper electrode 13. Thepiezoelectric thin film layer 12 with a thickness of more than 100 μmtakes longer time to form.

Further, in order to keep a good piezoelectric property, it is desirablethat the piezoelectric thin film layer 12 have a dipole orientationdegree of not less than 75%, more desirably, not less than 90%. This isbecause a dipole orientation degree of less than 75% causes an apparentpiezoelectric constant to be not more than one half of that when adipole orientation degree is 100%, and therefore causes a piezoelectricproperty of the piezoelectric thin film layer 12 to deteriorate, therebymaking it impossible to detect pressure satisfactorily. A dipoleorientation degree of not less than 75% ensures a sufficientpiezoelectric property.

In order to ensure a dipole orientation degree of not less than 75%, itis necessary to cause a first atom to be easily orientable when acrystalline column deposits, and it is desirable that a material for thebase layer 11 be a metal of the same component as a material for thepiezoelectric thin film layer 12 (e.g., the base layer 11 be made of Alwhen the piezoelectric thin film layer 12 is made of AlN; the base layer11 be made of Zn when the piezoelectric thin film layer 12 is made ofZnO). When the base layer 11 is formed of plural layers, it is desirablethat a top layer (a layer which comes into contact with thepiezoelectric thin film layer 12) be made of a metal of the samecomponent as the piezoelectric thin film layer 12.

Further, in the present embodiment, the metal diaphragm 10 is used aspressure transmission means, but it is possible to provide apiezoelectric sensor having an excellent heat resistance and requiringno cooling means, even with a conventional arrangement in which pressureis transmitted from a metal diaphragm through another member, such as apressure transmission bar, to a piezoelectric thin film layer.

Note that, the piezoelectric sensor of the present invention is appliedfor example to measuring internal pressure of an internal-combustioncylinder, but is not limited to this. The piezoelectric sensor of thepresent invention can be applied also to measuring a pressurefluctuation of a high-temperature, high-pressure fluid in a pipe and atank in a plant such as an atomic power plant.

Note that, the present invention can be arranged also as the followingpiezoelectric sensor:

(i) A first piezoelectric sensor including a piezoelectric thin filmelement stored in an axial hollow provided in a main metal body, themain metal body being mounted on an internal-combustion cylinder, theaxial hollow having a detection opening positioned in the cylinder,wherein:

the first piezoelectric sensor is constituted only of a metal diaphragmserving as pressure transmission means with respect to the piezoelectricthin film element.

(ii) The first piezoelectric sensor, wherein:

the piezoelectric thin film element is made of a piezoelectric thin filmmaterial having no Curie point and has a thickness of 0.1 μm to 0.1 mm.

(iii) The first piezoelectric sensor, wherein:

the piezoelectric thin film element is made of a thin film of aluminumnitride or zinc oxide.

(iv) The first piezoelectric sensor, wherein:

the piezoelectric thin film is made of a piezoelectric thin film ofaluminum nitride having a dipole orientation degree of not less than75%.

Third Embodiment

One embodiment of the present invention will be described below withreference to FIG. 8.

The piezoelectric sensor of the present invention is arranged so that:on a substrate 31, a base layer (first conductor film layer) 32, apiezoelectric thin film layer (piezoelectric element) 33, and an upperelectrode (second conductor film layer) 34 are laminated in this order.

As with the Second Embodiment, each of the films can be formed by aphysical vapor deposition (PVD) process.

FIG. 8 is a cross-sectional view of a thin-film piezoelectric sensoraccording to one embodiment of the present invention. The thin-filmpiezoelectric sensor is arranged so that: on the substrate 31, the baselayer 32 serving also as a lower electrode, the piezoelectric thin filmlayer 33, and the upper electrode 34 are laminated in this order.

The piezoelectric sensor is used with a lower surface of the substrate31 mounted on a target object. When an oscillation occurs in the targetobject, the oscillation is transmitted to the substrate 31. Whereas thesubstrate 31 oscillates together with the target object, a side of thepiezoelectric sensor opposite to the target object oscillates with delaydue to inertial force, and the piezoelectric thin film layer 33 issubjected to a compressive stress or tensile stress proportional tooscillatory acceleration. Further, a potential or a voltage proportionalto the stress is generated on both sides of the piezoelectric thin filmlayer 33 and is extracted (taken out) by the base layer 32 and the upperelectrode 34 provided on both sides of the piezoelectric thin film layer33. A measurement of the electrical output so extracted makes itpossible to detect a size of the oscillation or the acceleration of thetarget object.

The substrate 31, subjected directly to oscillation and pressure so asto generate a stress, can be constituted of an insulative or conductivesubstrate.

The insulative substrate is a substrate made of an oxide, carbide,nitride, or boride ceramic sintered body or quartz glass. Particularly,a substrate made of SiC (polycrystalline silicon carbide) is desirable.However, carbide ceramic substrates (e.g., substrates made of B₄C, TiC,WC, ZrC, NbC, and HfC), oxide ceramic substrates (e.g., substrates madeof AlO₃, ZrO₂, TiO₂, and SiO₂), nitride ceramic substrates (e.g.,substrates made of CBN, AlN, and TiN), and boride ceramic substrates(e.g., substrates made of TiB₂, ZrB₂, CrB₂, and MoB) can be used. Theseceramic materials are expected to have an excellent heat resistance, beeasy to produce, be inexpensive, have a high degree of hardness, andhave high density.

It is desirable that the conductive substrate be made of aheat-resistant metal material equivalent for a example to Inconel orSUS630, and a surface thereof be mirror-finished by a polishing orchemical method in order to prevent the piezoelectric thin film layer 33from cracking and peeling and enhance an orientation of a crystal axisof the piezoelectric thin film layer 33.

The base layer 32 is a buffer layer between the piezoelectric thin filmlayer 33 produced thereon and the substrate 31. The base layer 32orients a polarity of the piezoelectric thin film layer 33, orients thecrystal axis of the piezoelectric thin film layer 33, and improveswettability with respect to the substrate 31. The base layer 11 may bemade of TiN, MoSi₂, Si₃N₄, Cr, Fe, Mg, Mo, Nb, Ta, Ti, Zn, Zr, W, Pt,Al, Ni, Cu, Pd, Rh, Ir, Ru, Au, or Ag, and may be constituted of asingle layer or two or more layers made of a plurality of materials.

The piezoelectric thin film layer 33 is subjected to a stress generatedby the substrate 31 to generate a charge or voltage in proportion to thestress.

A material for the piezoelectric thin film layer 33 is preferablyaluminum nitride (AlN) or zinc oxide (ZnO), but is not limited to this,and only needs to be a piezoelectric material having no Curie point. Thepiezoelectric material having no Curie point does not lose apiezoelectric property until a crystal thereof is melted or sublimated.The piezoelectric material having no Curie point is for example asubstance having a wurtzite structure, namely GaN as well as AlN andZnO. Such a substance having a wurtzite structure has no symmetricproperty and therefore has a piezoelectric property. Further, thesubstance is not a ferroelectric substance and therefore has no Curiepoint. Therefore, the piezoelectric thin film layer 33 made of thepiezoelectric material has an excellent durability, does not deterioratein a piezoelectric property, and does not lose its function as apiezoelectric element even when exposed to a high temperature of 500° C.as in a cylinder of an engine. This makes it unnecessary to provide thepiezoelectric thin film layer 33 with cooling means or install thepiezoelectric thin film layer 33 in a low-temperature place, therebysimplifying the structure of the piezoelectric sensor.

Further, it is preferable that the piezoelectric thin film layer 33 hasa dipole orientation degree of not less than 75%, more preferably, notless than 90%. This is because a dipole orientation degree of less than75% causes an apparent piezoelectric constant to be not more than onehalf of that when a dipole orientation degree is 100%, and thereforecauses a piezoelectric property of the piezoelectric thin film layer 12to deteriorate, thereby making it impossible to detect pressuresatisfactorily. A dipole orientation degree of not less than 75% ensuresa sufficient piezoelectric property.

In order to ensure a dipole orientation degree of not less than 75%, itis necessary to cause a first atom to be easily orientable when acrystalline column deposits. Meanwhile, a piezoelectric material havingno Curie point is different from a ferroelectric substance such as leadzirconate titanate and cannot be controlled by an external electricfield after a crystal of the piezoelectric material has been formed.Therefore, in order to ensure that the piezoelectric thin film layer 33has a dipole orientation degree of not less than 75%, a crystal of thepiezoelectric thin film layer 33 needs to be controlled when thepiezoelectric thin film layer 33 is formed so that a dipole orientationdegree is not less than 75%. Specifically, when the piezoelectric thinfilm layer 33 is formed after the substrate 31 is provided with the baselayer 32 for arranging a dipole orientation of a crystal of apiezoelectric layer, a dipole orientation degree of the piezoelectricthin film layer 33 can be turned up by setting an optimum substratetemperature, an optimum distance between targets, and an optimum gaspressure and arranging a c-axis orientation of the crystal. Thus, inorder to improve a piezoelectric property, it is desirable to orient acrystal of a piezoelectric element in a c-axis direction.

Moreover, with an arrangement in which a side of the base layer 32 incontact with the piezoelectric thin film layer 33 is coated with a metal(Al, when the piezoelectric thin film layer 33 is made of AlN; Zn, whenthe piezoelectric thin film layer 33 is made of ZnO) contained in thepiezoelectric thin film layer 33, a dipole orientation degree of thepiezoelectric thin film layer 33 can be turned up even higher. At thistime, when the base layer 32 is constituted of plural layers, it isdesirable that a top layer (a layer which comes into contact with thepiezoelectric thin film layer 33) be made of a metal contained in thepiezoelectric thin film layer 33. Note that, a dipole orientation degreeis defined as a percentage at which crystalline columns having the samepolarity (positive or negative) occupy a surface of the piezoelectricthin film layer.

The upper electrode 34 detects a charge generated by an applied stressand can be made of the same material as the base layer 32, but notnecessarily. The upper electrode 34 may be made of a materialappropriately chosen in accordance with its compatibility with thepiezoelectric thin film layer 33, and may be constituted of a singlelayer.

Further, it is desirable that the piezoelectric thin film layer 33 ofthe thin-film piezoelectric sensor of the present invention have athickness of 0.1 μm to 100 μm. Further, it is more preferable that thepiezoelectric thin film layer 33 have a thickness of 0.5 μm or more to20 μm or less. It is even more preferable that the piezoelectric thinfilm layer 33 have a thickness of 1 μm or more to 10 μm or less. Thepiezoelectric thin film layer 33 with a thickness of less than 0.1 μm isprone to short-circuit the base layer 32 and the upper electrode 34. Thepiezoelectric thin film layer 33 with a thickness of more than 100 μmtakes longer time to form.

Note that, the present invention can be arranged as the followingthin-film piezoelectric sensors:

(i) A first high-temperature thin-film piezoelectric sensor having aninsulative substrate with a metal electrode, a piezoelectric ceramicthin film, and a further metal electrode laminated in this order, theinsulative substrate being made of an oxide, carbide, nitride, or borideceramic sintered body or quartz glass, the piezoelectric ceramic thinfilm being made of a piezoelectric thin film material having no Curiepoint, the piezoelectric ceramic thin film having a dipole orientationdegree of not less than 90%.

(ii) A second high-temperature thin-film piezoelectric sensor having aconductive substrate with a metal electrode, a piezoelectric ceramicthin film, and a further metal electrode laminated in this order, theconductive substrate being made of a heat-resistant metal materialequivalent for example to Inconel or SUS630, the metal electrode servingas a buffer layer, the piezoelectric ceramic thin film being made of apiezoelectric thin film material having no Curie point, thepiezoelectric ceramic thin film having a dipole orientation degree ofnot less than 90%.

(iii) The piezoelectric thin film sensor according to the first andsecond high-temperature thin-film piezoelectric sensors, wherein:

the piezoelectric thin film element is made of a piezoelectric thin filmelement with a thickness of 0.1 μm to 0.1 mm.

(iv) The piezoelectric thin film sensor according to the first andsecond high-temperature thin-film piezoelectric sensors, wherein:

the piezoelectric thin film element is made of a thin film of aluminumnitride or zinc oxide.

(v) The piezoelectric thin film sensor according to the first and secondhigh-temperature thin-film piezoelectric sensors, wherein:

the piezoelectric thin film layer has a piezoelectric ceramic thin filmand a metal electrode formed thereon, the metal electrode being dividedinto two or more.

Example 2

An example of the piezoelectric sensor according to the Third Embodimentwill be described below.

A base layer of a circular aluminum thin film with a diameter of 3 mmwas formed by a sputtering process on a surface of a quartz glasssubstrate with a diameter of 17 mm and a thickness of 1 mm. Further onthe base layer, a piezoelectric thin film layer of an AlN (aluminumnitride) thin film with a thickness of about 1 mm was produced by asputtering process.

An analysis of an X-ray diffraction pattern showed that the AlN has anexcellent crystalline property and is oriented in a c-axis direction.Further, a dipole orientation degree of a piezoelectric layer was 92%.

Next, a circular aluminum electrode, serving as an upper electrode, witha diameter of 3 mm was produced by a sputtering process on a surface ofthe AlN so as to overlap a lower electrode.

FIG. 10 shows a result of an oscillation detection measurement by acompression-type thin-film piezoelectric sensor using the thin-filmpiezoelectric sensor. The horizontal axis represents time; the verticalaxis represents a voltage of electricity generated. The measurement wascarried out as follows. The thin-film piezoelectric sensor was fixed ona metal structure. The metal structure was given a shock by a hammer ata point of 1.51 seconds in the time of the horizontal axis. Anoscillation generated by the shock was detected by the thin-filmpiezoelectric sensor. According to FIG. 10, the piezoelectric sensorgenerates a large voltage at a point of 1.519 seconds, i.e., at aboutthe same time as the shock. This shows that the thin-film piezoelectricsensor generates the voltage in response to the oscillation. That is,the thin-film piezoelectric sensor has an appropriate piezoelectricproperty.

Fourth Embodiment

One embodiment of the present invention will be described below withreference to FIG. 9.

The thin-film piezoelectric sensor of the present invention is arrangedso that: on the substrate 31, the base layer (first conductor filmlayer) 32, the piezoelectric thin film layer (piezoelectric element) 33,and a plurality of divided upper metal electrodes (second conductor filmlayer) 35 are laminated in this order.

Materials and producing methods of the substrate 31, the base layer 32,and the piezoelectric thin film layer 33 are the same as those in theThird Embodiment. However, the piezoelectric thin film layer 33 isprovided with the divided metal electrodes 35, which are divided intotwo or more.

A material and producing method of the divided upper metal electrodes 35is substantially the same as those in the Third Embodiment, except thatthe divided upper metal electrodes 35 are formed with a pattern mask andthe like after the piezoelectric thin film layer is formed. That is,whereas the upper electrode 34 is formed as a monolithic layer in theThird Embodiment, the divided upper metal electrodes 35 divided intoarbitrary shapes and numbers is produced in the Fourth Embodiment byplacing an arbitrary pattern mask on a surface of the piezoelectric thinfilm layer 33 on the substrate 31.

With such an arrangement, when a stress varying from place to place isgenerated on a surface of the thin-film piezoelectric, the stress suchas pressure varies depending on positions of the divided upperelectrodes 35. This makes it possible to detect a difference betweendifferent charges and voltages generated on the divided upper electrodes35. That is, it is possible to detect which part of the thin-filmpiezoelectric sensor is subjected to the stress.

Such a thin-film piezoelectric sensor can be used to detect anoscillatory direction by measuring a fluctuation in a temporal stressdistribution. Further, in case of arranging a cantilever-type or adiaphragm-type thin-film piezoelectric sensor, it becomes possible todetect a difference with a hardware device by detecting a difference ofthe stress, thereby achieving high-sensitivity detection of a shearstress without restrictions placed on a dynamic range of an amplifier.

The invention being thus described, it will be obvious that the same waymay be varied in many ways. Such variations are not to be regarded as adeparture from the spirit and scope of the invention, and all suchmodifications as would be obvious to one skilled in the art are intendedto be included within the scope of the following claims. For example,also embodiments obtained by combining the technical means respectivelydisclosed in different embodiments are included in the technical scopeof the present invention.

INDUSTRIAL APPLICABILITY

As described above, the piezoelectric sensor according to the presentinvention can be applied to a transparent piezoelectric sensor used in atransparent input device.

The piezoelectric sensor according to the present invention is arrangedso as to include: a transparent piezoelectric element having apiezoelectric property; and a pair of transparent conductor film layersopposed to each other with the piezoelectric element therebetween,wherein the transparent piezoelectric element and the transparentconductor film layers are formed between a pair of transparentsubstrates opposite to each other.

The piezoelectric sensor of the present invention has a pair oftransparent conductor films opposed to each other with a piezoelectricelement interposed therebetween. Further, when one of the pair ofsubstrates is subjected to outside pressure, the piezoelectric elementtakes a charge, and the charge is detected by the transparent conductorfilms, so that the outside pressure is detected. Therefore, thetransparent conductor films do not need to come into contact with eachother, thereby bringing about an effect of preventing a flaw fromoccurring due to contact. Moreover, the transparent conductor films donot need to be deformed, thereby bringing about an effect of providing apiezoelectric sensor having an excellent durability as compared with theconventional arrangements.

Such a piezoelectric sensor can be applied to an input device. In aninput device having a plurality of the piezoelectric sensors of thepresent invention, i.e., a transparent input device having a pluralityof the piezoelectric sensors of the present invention serving as akeypad, a pair of transparent conductor film layers of each of thepiezoelectric sensors do not come into contact with each other, so thata flaw due to contact can be prevented and the piezoelectric sensor hasan excellent durability. Therefore, the input device having thepiezoelectric sensor of the present invention also brings about aneffect of providing an input device having the same effect. Further,outside pressure is detected by the piezoelectric element, therebybringing about an effect of detecting a position under pressure with asimple structure.

Further, the piezoelectric sensor of the present invention withstandshigh temperatures and therefore can be applied to a piezoelectric sensorused in an internal-combustion engine and an atomic power plant.

The piezoelectric sensor of the present invention has the piezoelectricelement made of a piezoelectric material having no Curie point (such asa substance having a wurtzite structure), i.e., a dipole orientationfilm of aluminum nitride, zinc oxide, or a piezoelectric material havingthe same effect. Such a piezoelectric material does not deteriorate in apiezoelectric property even at high temperatures, so that thepiezoelectric element does not need to be cooled by cooling means likethe conventional arrangements. Further, it is no longer necessary toinstall the piezoelectric element in a low-temperature place, therebyeliminating the need for a pressure transmission bar and the like andbringing about an effect of simplifying the structure of thepiezoelectric sensor.

Particularly, the piezoelectric sensor can be applied to a piezoelectricsensor having the pressure detection means stored in an axial hollowprovided in a main metal body, the main metal body being mounted on aninternal-combustion cylinder, the axial hollow having a detectionopening positioned in the cylinder. In this case, the piezoelectricelement has heat resistance, so that the structure of the piezoelectricelement can be simplified effectively. Further, with a dipoleorientation degree of not less than 75%, the piezoelectric elementretains a piezoelectric property, and the piezoelectric sensor functionssatisfactorily.

Further, when the piezoelectric sensor of the present invention isarranged so that the piezoelectric element is formed by a physical vapordeposition (PVD) process on a metal diaphragm to be press-fitted intothe opening of the axial hollow in the main metal body, the diaphragmand the piezoelectric element become very thin, so that the pressuredetection means becomes thin and small. Further, the metal diaphragm andthe piezoelectric element strained so slightly, thereby eliminating theneed for an arrangement for preventing a strain. This brings about aneffect of extremely simplifying the structure of the piezoelectricsensor having heat resistance.

Further, the thin-film piezoelectric sensor of the present invention canbe used as a thin-film piezoelectric sensor having a substrate with afirst conductor film layer, a piezoelectric element, and a secondconductor film layer laminated on a surface thereof in this order.

With the foregoing arrangement, a small, inexpensive thin-filmpiezoelectric sensor for detecting acoustic emission and oscillation oracceleration in an engine and an atomic power plant can be providedwhich ensures a piezoelectric property, requires no cooling means, andhas an excellent durability.

The piezoelectric sensor of the arrangements described above bringsabout an effect of providing a market with a small, inexpensivepiezoelectric sensor having an excellent heat resistance and durability.

1. A piezoelectric sensor, comprising: a transparent piezoelectricelement having a piezoelectric property; and a pair of transparentconductor film layers opposed to each other with the piezoelectricelement therebetween, the transparent piezoelectric element and thetransparent conductor film layers being formed between a pair oftransparent substrates, opposed to each other, which serve as pressuretransmission means.
 2. The piezoelectric sensor according to claim 1,wherein said piezoelectric element is made of aluminum nitride or zincoxide.
 3. The piezoelectric sensor according to claim 1, wherein saidpiezoelectric element has a thickness of 1 μm to 10 μm.
 4. Thepiezoelectric sensor according to claim 1, wherein a further transparentconductor film layer is formed on one of the pair of transparentsubstrates so as to be positioned in a side opposite to the transparentconductor film layer.
 5. A method of producing a piezoelectric sensor,comprising the steps of: forming transparent conductor film layersrespectively on a pair of transparent substrates; forming a transparentpiezoelectric element having a piezoelectric property so as to cover oneof the transparent conductor film layers which has been formed on one ofthe pair of transparent substrates; and bonding the piezoelectricelement to the other of the transparent conductor film layers which hasbeen formed on the other transparent substrate not provided with thepiezoelectric element.
 6. An input device, comprising a plurality ofpiezoelectric sensors each of which is the piezoelectric sensoraccording to claim
 1. 7. A piezoelectric sensor, comprising pressuredetection means which includes: pressure transmission means fortransmitting pressure; and a piezoelectric element for receiving thepressure so as to convert the pressure into an electrical signal, thepressure being transmitted from the pressure transmission means, saidpiezoelectric sensor being characterized in that said piezoelectricelement is made of a piezoelectric material having no Curie point andhas a dipole orientation degree of not less than 75%.
 8. Thepiezoelectric sensor according to claim 7, wherein said piezoelectricelement is made of a substance having a wurtzite structure.
 9. Thepiezoelectric sensor according to claim 8, wherein said piezoelectricelement is made of aluminum nitride or zinc oxide.
 10. The piezoelectricSensor according to claim 7, wherein said piezoelectric element isformed by a physical vapor deposition process.
 11. The piezoelectricsensor according to claim 7, wherein said piezoelectric element has athickness of 0.1 μm or more to 100 μm or less.
 12. The piezoelectricsensor according to claim 7, wherein: said pressure transmission meansis constituted of a metal diaphragm, and said pressure detection meansis formed by providing a piezoelectric element on a surface of the metaldiaphragm.
 13. The piezoelectric sensor according to claim 7, furthercomprising a main metal body for mounting said pressure detection meanson an internal-combustion cylinder, the main metal body having an axialhollow for connecting an inside of the cylinder with an outside of thecylinder, said pressure detection means being provided in the axialhollow.
 14. The piezoelectric sensor according to claim 7, wherein: saidpressure transmission means is a substrate, and a first conductor filmlayer, said piezoelectric element, and a second conductor film layer arelaminated on a surface of the pressure transmission means in this order.15. The piezoelectric sensor according to claim 14, wherein saidsubstrate is an insulative substrate made of an oxide, carbide, nitride,or boride ceramic sintered body or quartz glass.
 16. The piezoelectricsensor according to claim 14, wherein said substrate is a conductivesubstrate made of a heat-resistant metal material.
 17. The piezoelectricsensor according to claim 14, wherein said first conductor film layerhas a surface, being in contact with the piezoelectric sensor, which iscoated with a metal contained in the piezoelectric sensor.
 18. Thepiezoelectric sensor according to claim 14, wherein said secondconductor film layer is divided into two or more.